Filter units, having two filter cartridges, are known as so-called duplex filters and are used particularly as lubricating oil filters for marine transmissions. Here, there is the technical regulation that it must be possible to perform a filter change with the machine running. For this case, a switching device is provided by means of which one filter is switched off and the other filter has a flow passing through it. In the meantime, the shut off filter can be exchanged. Due to the two identical filter cartridges, such duplex filters are relatively large, heavy and expensive.
So-called gap-type filters are also known that do not need to be exchanged because they can be cleaned during operation. A disadvantage with the gap-type filters is that a maximum grade of filtration cannot be attained. The gap-type filter has a grade of filtration limit of approximately 50 μm. The grade of filtration is determined by the gap width; there is no filtering in the depth, rather filtration occurs only in a filter plane.
The GB 1,164,628 document discloses a duplex filter for a marine transmission, thus a lubricating oil filter. Such known duplex filters have two identical filter cartridges, which can be populated with different filter elements, for example, with a paper filter element or fine filter element, or a coarse filter element in the form of a sieve filter. Here, the fine filter is provided for the time period during which the marine machine is being broken-in. After this initial break-in phase, the fine filter is replaced by the coarse filter, a fine filter is no longer used after the initial break-in phase. The exchange from the fine filter to the coarse filter occurs by actuating a switching device, which allows the exchange of the fine filter cartridge while the machine is running.